1. Field of the Invention
The present invention relates to a transfer-separation device and an image forming apparatus.
2. Description of the Related Art
As a transfer device in an image forming apparatus, a device using an endless-belt-like intermediate transfer member (hereinafter, “intermediate transfer belt”) has been known. In a color image forming apparatus, a device is widely used that primarily transfers toner images each having a different color onto the intermediate transfer belt from an image carrier such that toner images are sequentially superimposed one on top of the other to form a full-color image and performs secondarily transfers the full-color image onto a recording medium from the intermediate transfer belt.
As a transfer device using an intermediate transfer belt, a recording medium is sandwiched between the intermediate transfer belt and a secondary transfer roller, and a transfer bias (or a transfer current) having a polarity opposite to that of a toner image is applied to the secondary transfer roller to form a transfer electric field in a direction along which the toner image is attracted to a recording medium side between the recording medium and the intermediate transfer belt. A device that uses this transfer electric field to transfer the toner image onto the recording medium from the intermediate transfer belt is known. In this transfer device, a constant current having a polarity opposite to that of the toner image is supplied to the secondary transfer roller to obtain stable transfer performance with respect to irregularities in a resistance of the secondary transfer roller due to, e.g., a change in an environment. However, a resistance is greatly reduced due to a change in a moisture content of the recording medium in a high-humidity environment. Of the current supplied to the secondary transfer roller, a current that escapes to a carrying member that is in contact with the recording medium is largely increased via the recording medium. Therefore, of the current supplied to the secondary transfer roller, a current that contributes to forming a transfer electric field between the recording medium and the intermediate transfer belt is reduced, thus greatly decreasing transfer performance.
To solve such a problem, the present inventors have proposed, in Japanese Patent Application Laid-open No. 2004-184875, a transfer device and an image forming apparatus including the transfer device. The transfer device sandwiches a recording medium between an intermediate transfer belt and a secondary transfer roller, and has a secondary-transfer-opposed roller arranged on a rear surface of the intermediate transfer belt. A transfer bias having the same polarity as that of a toner image is applied to the secondary-transfer-opposed roller to transfer the toner image onto the recording medium from the intermediate transfer belt. In the conventional transfer device, a current having the same polarity as that of a toner image is supplied to the secondary-transfer-opposed roller provided on the rear surface of the intermediate transfer belt to form a transfer electric field in a direction along which the toner image recoils with respect to the intermediate transfer belt between the intermediate transfer belt and the recording medium. This transfer electric field is used to transfer the toner image onto the recording medium from the intermediate transfer belt. When the secondary-transfer-opposed roller is used to supply a constant current from the intermediate transfer belt side, even if a resistance of the recording medium is reduced due to fluctuations in an environment, the supplied current first forms a transfer electric field between the intermediate transfer belt and the recording medium, and then flows through the recording medium. Thus, the transfer electric field formed between the intermediate transfer belt and the recording medium hardly becomes under the influence due to a change in a resistance of the recording medium, and can be stably formed. Therefore, constantly stable transfer performance can be obtained.
In the conventional transfer device, the secondary-transfer-opposed roller applies a bias having the same polarity as that of the toner image, and transfers the toner image onto the recording medium from the intermediate transfer belt. Therefore, the secondary-transfer-opposed roller has a function as a repulsive roller. In this example, when a resistance of the secondary-transfer-opposed roller (repulsive roller) is increased and a resistance of the secondary transfer roller is set low, a current that leaks through the intermediate transfer belt is no longer present, and the current applied to the repulsive roller directly becomes a transfer current flowing toward the recording medium from the intermediate transfer belt, thus stabilizing a transfer ratio.
The present inventors have also proposed, in Japanese Patent Application Laid-open No. 2005-181863, a transfer-separation device and an image forming apparatus including the transfer-separation device. The transfer-separation device includes a charge-eliminating/separating device that eliminates charges from a recording medium and separates the recording medium from an intermediate transfer belt after secondary transfer. In the conventional transfer-separation device, when 0 microampere or a separation bias obtained by superimposing an alternating current (AC) on a constant-current-controlled direct current (DC) having a polarity opposite to that of a toner and a value far smaller than a secondary transfer bias is applied to a charge-eliminating/separating needle placed at a position closer to a secondary transfer roller than the intermediate transfer belt, an abnormal image due to discharge for separation/charge elimination can be avoided, and an interference of a current and a secondary transfer current due to discharge for separation/charge elimination can be suppressed. Accordingly, stabilizing a transfer ratio is stabilized.
In the former conventional technology, when a resistance of the secondary-transfer-opposed roller (repulsive roller) is increased and a resistance of the secondary transfer roller is set low, a current applied to the repulsive roller is prevented from leaking to, e.g., a roller that stretches the intermediate transfer belt through the intermediate transfer belt, and the current applied to the repulsive roller all becomes a transfer current flowing toward the recording medium from the intermediate transfer belt, thus obtaining a stable transfer ratio. In the latter conventional technology, a charge eliminating current discharged to the recording medium from the charge eliminating member, e.g., a charge eliminating needle does not affect a transfer current flowing toward the recording medium from the intermediate transfer belt. Accordingly, a stable transfer ratio can be achieved.
However, an electroconductive small foreign matter (e.g., a carbon fiber with a diameter of approximately 10 micrometers used for, e.g., a charge eliminating blush that is provided in a recording-medium conveying path in the image forming apparatus to eliminate charges from the recording medium) adhering to the recording medium is attached to, e.g., a high-resistance or an insulating guide plate provided between the charge eliminating member and the secondary transfer roller at on rare occasions. It can be considered that this phenomenon occurs since the foreign matter is drawn by an electric field that produces discharge from the charge eliminating member to the recording medium. An insulating resin or air alone is assumed to enter a space between the charge eliminating member and the secondary transfer roller. However, when the electroconductive foreign matter enters the space between the charge eliminating member and the secondary transfer roller, a spatial distance between the charge eliminating member and the secondary transfer roller is shortened at a position of the foreign matter alone, and an electric field in the space is increased. Therefore, abnormal discharge concentrated on the position of the foreign matter occurs, and a power supply that supplies a high voltage to the charge eliminating member abnormally stops. When the power supply is not rapidly subjected to abnormal stop, the small foreign matter is heated, and a peripheral insulating resin having the foreign matter adhering thereto may be eventually molten. Joule heat generation due to a flow of an abnormal discharge current through the foreign matter is considered as a cause of this phenomenon. When the apparatus abnormally stops or normally stops after continuation of an operation until the end and then abnormal discharge stops, the molten resin is cooled and again solidified. However, at this moment, the foreign matter is taken in and the resin is hardened. Therefore, the foreign matter is fixed in the resin and cannot be separated from the same. Then, abnormal discharge continuously occurs every time the apparatus operates.
When the foreign matter is fixed in the resin of the guide plate, maintenance of the apparatus cannot be completed simply by removal of the foreign matter based on a cleaning operation, and the molten member or a unit including this member must be discarded and replaced with a new one. Therefore, an operation time for maintenance is increased to raise a labor cost, and wastefully discarding an article leads to deterioration in an environment.